Topic on Extension talk:AdvancedSearch/Flow

Unclear why this is needed.

Is it possible to remove the namespace pane (force it not to load) and keep the advanced search pane?

At the moment there is no setting. It should be relatively simple to implement, maybe you can propose such a setting on Phabricator?

As a workaround for your own installation, you can edit the file modules/ext.advancedSearch.init.js and remove/comment out the lines

$advancedSearch.append( buildNamespacesPaneElement(
   state,
   headerContainer,
   namespacePresets,
   namespaceSelection,
   searchableNamespaces
) );

That will disable the namespaces without ill effects.